The Terminator has been one of the most long running science fiction/action franchises to ever exist. Starting in the '80s, the series has seen ups and downs and has extended into our present decade with middling results. The last entry, Genisys, was mostly bashed by the critics and received a lukewarm welcome from long running fans of the cybernetic brand. Although the film did some decent global box office receipts, the studio has completely cooled to the idea of continuing on with another film in the saga and have deleted the upcoming project from their schedule. There is no longer any planned Terminator 6. Alongside this news, Arnold is officially done playing the character and has no interest in returning.
Although this is an ending for what could have been a great continuing story, the originator James Cameron, is still interested in retooling or rebooting the franchise at some point. However, he is wrapped up in his Avatar series right now and may not have the time to bring back a worthy successor to the original two adventures.